Field
The present disclosure relates to an optical filter configured to transmit light of a predetermined wavelength.
Description of the Related Art
In these years, an optical filter has come to be known in which apertures are periodically arranged in a metal thin film so that light of a predetermined wavelength is mainly transmitted (refer to Japanese Patent No. 3008931). Bethe's theory of diffraction has been used to explain a phenomenon occurring upon light irradiation of a conductive thin film having sub-wavelength apertures. However, such a conductive thin film can transmit light of a predetermined wavelength that is larger than the diameter of the apertures. Accordingly, the transmittance of the conductive thin film is more than expected on the basis of the total area of the apertures formed therein. When the conductive thin film transmits light of a predetermined wavelength in accordance with the period of the apertures, plural transmission subpeaks are observed on the short-wavelength side of the maximum transmission peak. Light at the maximum transmission peak (main peak) is light propagated by an abnormal transmission phenomenon caused by surface plasmons. Other light at the transmission subpeaks is light propagating through apertures (cylindrical waveguides). Thus, the conductive thin film transmits light of the predetermined wavelength and also light of other wavelengths. The wavelength selectivity of such an optical filter needs to be enhanced.
The surface plasmons upon irradiation of the conductive thin film with incident light occur in the surface of the conductive thin film. The frequency of the surface plasmons depends on the period of the apertures. A continuous surface (through hole) needs to extend from the Incident surface to the output surface in order to propagate surface plasmons. The surface plasmons on the output-surface side are output by energy diffraction of near-field light caused on the aperture end. Thus, the electric field distribution has high intensity on the aperture end.
The other propagation light (sometimes referred to as waveguide mode) passing through each aperture considered as a waveguide reflects off the inner wall of the aperture to propagate in the center of the aperture. Accordingly, the electric field has the highest intensity in the center of the cylindrical aperture. On the basis of such a phenomenon, a technique for enhancing wavelength selectivity has been proposed in which a second conductive thin film is disposed inside the aperture so that waveguide mode light is not transmitted (refer to Japanese Unexamined Patent Application Publication No. 2010-160212).
A laminated filter has also been proposed in which the first and second conductive thin layers patterned identically are laminated with such an interlayer distance therebetween that no optical near-field interaction occurs. This laminated filter transmits light within a narrow band, compared with a monolayer filter (refer to Japanese Patent No. 4995231).
In the above-described optical filters in Japanese Unexamined Patent Application Publication No. 2010-160212 and Japanese Patent No. 4995231, the half width needs to be narrowed for practical use to achieve high wavelength selectively. In addition, when such an optical filter is produced so as to transmit visible light for application to, for example, an image-pickup element, the diameter of the apertures needs to be decreased to about half of that in Examples of Japanese Unexamined Patent Application Publication No. 2010-160212. In this case, it becomes difficult to form the second conductive thin films inside the apertures.